Ground source heat pump systems, also known geothermal or geoexchange systems, have been used for heating and cooling buildings for more than half a century. More than ten years ago, the Environmental Protection Agency evaluated all available heating and cooling technologies and concluded that ground source heat pump systems were the most energy efficient systems available in the consumer marketplace.
Conventional ground source heat pump systems operate on a simple principle. In the heating mode they collect heat energy from the ground and transfer it to a heat pump, which concentrates the heat and transfers it to a building's heat distribution system which in turn heats the building. In the cooling mode, heat from the building is collected by the cooling system and transferred to the heat pump, which concentrates the energy and transfers it to a ground source loop, which transfers the heat to the ground. In both modes, only a small amount of the heat comes from the electricity that runs the compressor; most of the heating and cooling energy comes from the ground. This allows ground source heat pump systems to achieve more than 100% efficiency: every unit of electrical energy consumed by the heat pump produces more useable heat than an electrical resistance heater can produce with the same unit of electricity.
Even though ground source heat pump systems achieve efficiencies of up to 350% compared to less than 100% for many conventional systems, they have been slow to penetrate the consumer marketplace because of high capital costs, high installation costs, difficult installation procedures and low energy cost savings due to historically low energy prices.
These high capital and installation costs have largely been due to fundamental inefficiencies in the ground loop subsystem. In a typical installation, the ground loop consists of hundreds or thousands of feet of looped plastic piping buried in deep trenches or deep holes drilled into the ground. An antifreeze solution such as glycol is pumped through this loop to absorb heat energy from the ground (in the heating mode) or transfer heat energy to the ground (in the cooling mode.) Few installations have sufficient available land for trenching so loops are most commonly installed in deep holes and this makes them relatively expensive for several reasons.
First, each loop consists of a supply and return line, which must fit down the same hole. With an outer diameter of an inch or more for each pipe and a tendency for these pipes to bow away from each other due to the plastic material's memory of being coiled for shipment, the hole typically needs to have a diameter of 4 to 6 inches to allow the loop to be installed. Holes of this size are relatively expensive to drill and require heavy equipment that disrupts landscaping, making it expensive to retrofit existing homes. Holes of this size also leave large voids around the loop that must be filled with materials such as bentonite clay in order for heat to transfer from the ground to the loop, which adds significantly to the cost of installation.
Second, having both supply and return lines in the same hole results in thermal “short circuiting” which reduces the efficiency of the loop. In the heating mode, for example, cool fluid from the heat pump absorbs heat from the ground as it goes down the supply line in the hole, cooling the ground around the pipe. When the warmed fluid comes back up the hole in the return line, it passes through the ground that was just cooled, losing some of the heat it has just picked up. This lowers the efficiency of the loop so the loop must be made longer to compensate, adding to the cost of drilling and piping.
Third, for the ground loop to function, the antifreeze solution must be pumped through hundreds or thousands of feet of small diameter piping. This consumes a significant amount of electric energy, lowering the overall efficiency of the system.
In recent years, a new ground source heat pump technology has evolved to overcome some of the inefficiencies of conventional systems. This technology, called “direct geoexchange,” replaces the conventional plastic ground loop with a small-diameter copper loop. Instead of an antifreeze solution, direct geoexchange systems pump a refrigerant through the loop to pick up heat from the ground or give off heat to the ground in the same way that conventional ground loops function.
Direct geoexchange has some significant advantages over conventional systems. First, the direct geoexchange loop runs directly to and from the heat pump's compressor, eliminating the heat exchanger that is required by conventional systems to transfer heat from the loop to the heat pump. Second, the small diameter of the direct exchange loop makes it possible for loops to be installed in smaller diameter holes in the ground; this reduces the cost of drilling and backfilling the holes and reduces the size of the drill rig required to drill the holes, decreasing damage to landscaping in retrofit applications. Third, the copper pipes used in direct geoexchange transfer heat more efficiently to and from the ground so the total length of loop required is typically less than conventional systems. Because of these improvements, direct geoexchange systems can be cheaper than conventional ground source systems and more energy efficient.
In spite of these inherent advantages, direct geoexchange also has some significant disadvantages. First, both supply and return pipes run in the same hole, so the thermal short circuit problems of conventional systems remain. Second, the loop system pumps much more refrigerant through many more feet of piping past many more connections than conventional systems, so the potential for refrigerant leaks is increased. Third, direct geoexchange requires large volumes of refrigerant to flow through the loop, behaving differently in the heating and cooling modes, and requiring additional refrigerant reservoirs and flow control systems to compensate. Fourth, changing from heating to cooling modes requires the system to have expensive and often unreliable reversing valves in the refrigerant lines. Because of these inefficiencies, direct geoexchange is only able to achieve a modest 15% improvement in total energy efficiency over conventional ground source heat pump systems.
Direct geoexchange and conventional ground source heat pump systems have additional limitations. Both require a significant amount of electrical power to pump fluids through hundreds or thousands of feet of piping. This not only limits overall system efficiency but also limits the environments in which it can be installed. This kind of power is not often available or reliable in the world's developing countries, so existing ground source heat pump systems have limited potential to penetrate broad world markets. In addition, since both systems are designed to heat and cool whole buildings, neither can efficiently be installed on the incremental room-by-room basis on which most of the world adopts heating and air conditioning.
In summary, conventional geoexchange systems and direct expansion geoexchange systems have significant limitations in energy efficiency, installation cost and installation flexibility.
There is a need for a geothermal exchange system that operates without a refrigerant loop, utilizes much less power than conventional refrigerant or coolant based geoexchange systems, results in lightweight heat exchangers, that can be configured in a wide range of interior locations, has an extended lifetime due to fewer parts and no circulating fluids, has reduced ground loop installation costs and provides enhanced cooling and heating efficiency compared to power used.